The present invention relates to pressure sensitive adhesive compositions. In particular, an acrylic emulsion polymer is employed as the polymeric component of a pressure sensitive adhesive formulation. The acrylic emulsion polymer has substantially Newtonian-like flow characteristics.
Pressure sensitive adhesives (PSAs) are a class of adhesive compositions which are applied with pressure, usually finger pressure, and generally do not undergo a liquid to solid transition in order to hold materials together. PSAs can be solvent-free natural or synthetic resins having a viscoelastic property termed tack. Tack is a property characterized by the rapid wetting of a substrate by a polymer to form an adhesive bond upon brief contact with the substrate under light pressure. Typical applications for PSAs include pressure-sensitive tapes, labels, decals, decorative vinyls, laminates, wall coverings and floor tiles.
The early pressure sensitive tapes used adhesives that were based on organic solvent solutions of natural or synthetic rubber, tackified by a resinous material. Later, the rubber in pressure sensitive adhesives was replaced with styrene-butadiene block copolymers. More recently, the polyacrylates have gained wide acceptance in pressure sensitive adhesive formulations due to their clarity and resistance to oxidation and sunlight.
Acrylic copolymer PSAs are available as solution or aqueous emulsion polymers. The solution polymers have the disadvantages of low molecular weight due to chain transfer during the polymerization, viscous solutions at higher molecular weights, and the need for elaborate coating processes and solvent recovery equipment to satisfy economic and environmental requirements. The anionic and nonionic emulsifiers generally employed in acrylic emulsion polymerization protect the soft pressure sensitive polymers from impact coalescence during the reaction and stabilize the latex for satisfactory mechanical and storage stability. However, the emulsifiers do have adverse effects on pressure sensitive adhesive properties. The emulsifiers employed in emulsion polymerization adversely effect water resistance, tack and adhesion properties of emulsion polymer adhesives.
Canadian Pat. No. 814,528, issued June 3, 1969, discloses low molecular weight alkali soluble resins, resin cuts and methods for their preparation and purification. The resins are disclosed as being especially useful as emulsifiers, leveling agents and film-formers. The number average molecular weight of the resins range from 700-5000 and the resins have acid numbers between 140 and 300. The resins are disclosed as emulsifiers in the preparation of emulsion polymers resulting in emulsion polymers which are stable and substantially free from coagulum. For use as an emulsifier in emulsion polymerization reaction, the resins must have a number average molecular weight between 1,000 and 2,000 and preferably between 1,000 and 1,500. Resins having a number average molecular weight greater than 2,000 result in unstable and coagulated emulsion polymers when used as the emulsifier in emulsion polymerization reactions.
The present invention provides advantages over known acrylic copolymer adhesives by providing an improved pressure-sensitive adhesive formulation having (a) fine particle size emulsions, (b) emulsion viscosities which can be varied from low to high with no sacrifice in stability, (c) emulsion viscosities which are stable under high shear conditions encountered in roll coating operations (Newtonian-like flow characteristics) and (d) low foam production which is desirable in roll coating operations.